Image sensors used in cell phone cameras, web cameras, and the like, may be implemented in a vertically stacked orientation, wherein a sensor die comprising the photosensitive element is stacked atop a circuitry die, which comprises processing and readout circuitry. Stacked or vertically integrated image sensors allow for larger photosensitive areas, which improves image quality, while maintaining a small footprint. Due to uneven power distribution, however, the circuitry die may experience thermal gradients across the die. Thermal gradients transferred to the sensor die may affect various temperature-dependent operating characteristics of the sensor die. For example, pixels exposed to increased temperatures may exhibit an increase of dark current, thereby reducing the signal-to-noise ratio. The non-uniform pixel response due to the thermal gradient across the pixel array may create image artifacts, such as image shading defects.
Electrically connecting the two dies may be achieved by etching vias, which are then filled with metal to form electrical interconnects, or by providing a metal-to-metal hybrid bond below the image sensor array. Conventional heat sinks formed from metal cannot be used in stacked implementations because, unless each interconnect is separated from the heat sink by a dielectric passivation, the metal heat sink would short the electrical interconnects. In applications where interconnects are under the pixel array, isolating each interconnect with a dielectric passivation will not completely remove the thermal gradient across the pixel array. In addition, in applications where interconnects are outside the pixel array, isolating each interconnect with a dielectric passivation will not completely remove the thermal gradient around the pixel array periphery.